1. Field of the Invention
The present invention relates to a writing instrument, and more particularly to a double-chuck mechanical pencil having a tip chuck located at the tip of a tip fitting to hold a lead and a delivery chuck for delivering the lead.
2. Description of the Related Art
A conventional example of a mechanical pencil having two chucks is described in Japanese Utility Model Application Laid-Open No. 4-119435.
Generally, a tip chuck and a delivery chuck of such a mechanical pencil are formed of a metal material. To manufacture the tip and delivery chucks, an approximately cylindrical metal material must be sawed, rubbed, and cut by a cutter to divide the material into a plurality of chuck elements (e.g., two or three), which then are subjected to secondary processing such as burr removal.
Subsequently, the chuck elements of the delivery chuck must be extended outwardly, while the chuck elements of the tip chuck must be subjected to secondary processing such as narrowing because excessively-opened chuck elements cause a lead to fall (e.g., drop-out from the pencil) when the tip chuck is moved forward to release the lead. Thus, manufacturing such a mechanical pencil is labor-intensive and costly.
In view of the foregoing and other problems of the conventional writing instruments, an object of the present invention is to provide a double-chuck mechanical pencil that reduces manufacturing costs.
To achieve this and other objects, a double chuck mechanical pencil according to the present invention includes an outer tube, a tip fitting provided in the front of the outer tube, a tip chuck located at the tip of the tip fitting for selectively holding a lead and urged rearwardly in the axial direction, and a delivery chuck for selectively delivering a lead, the delivery chuck being movable forwardly to deliver a lead after the tip chuck has been moved forward selectively to release the lead, wherein at least one of the tip chuck and the delivery chuck is formed of a synthetic resin material.
By forming the chucks of a synthetic resin material into a desired shape, secondary processing such as burr removal or the widening or narrowing of the tip of the chuck, or the like, is unnecessary, thereby reducing manufacturing costs.
Preferably, the synthetic resin material includes polyoxymethylene, and preferably consists essentially of polyoxymethylene. This composition provides a mechanically strong chuck with high abrasion resistance.
Additionally, preferably, the synthetic resin material includes, and preferably consists essentially of, polyoxymethylene in which glass fibers are mixed. This composition provides elastic chucks which reduce damage to leads being held by the chucks, thereby preventing the leads from being broken. Appropriate elasticity can be provided for the chucks by mixing polyoxymethylene with substantially about 5 to about 20 wt %, and preferably, substantially about 15 wt % of glass fibers.
Alternatively, the synthetic resin material includes, and preferably consists essentially of polyoxymethylene in which a lubricant is mixed. This composition enables smooth sliding between the tip fitting and the tip chuck, and between the delivery chuck and its peripheral members. Specific lubricants include molybdenum, silicon, tungsten, and/or the like.
Additionally, preferably, at least the one of the tip and delivery chucks that is formed of the synthetic resin material includes a plurality of chuck elements obtained by dividing the head of the chuck along the circumferential direction, and the circumferential gap between the adjacent chuck elements, formed when the plurality of chuck elements are allowed to approach one another such that the contour of the external shape of the chuck elements forms a true circle as seen from the axial direction, is substantially about 0.15 mm or less.
In the conventional chuck formed of a conventional metal material, an approximately cylindrical chuck is divided into a plurality of chuck elements by a cutter, so that the circumferential gap between the chuck elements cannot be reduced below the edge width of the cutter (e.g., about 0.15 to 0.2 mm). However, by forming the chucks of the synthetic resin, the chucks of the present invention can be formed into desired sizes by molding. Accordingly, the circumferential gap between the plurality of chuck elements formed when the chuck elements are allowed to approach one another such that the contour of the external shape of the chuck elements forms a true circle as seen from the axial direction, can be set to a small value (e.g., substantially about 0.15 mm or less). Consequently, the area in which the lead held by the chuck elements contacts the chuck elements can be increased to allow the leads to be reliably sandwiched by the chuck elements.
Additionally, preferably, the tip and delivery chucks include a plurality of chuck elements that are formed by dividing the chucks along the circumferential direction from a first to a second side (e.g., their head to rear side) and that are mutually separated and opposed, and the axial length of the separated and opposed chuck elements of the tip chuck is set to be shorter than that of the delivery chuck.
This configuration enables the mutual extension of the chuck elements of the tip chuck to be reduced below that of the delivery chuck. Thus, if the lead becomes short and is located in front of the delivery chuck, then the inventive mechanical pencil provides a lead holding force sufficient to prevent the chuck elements of the tip chuck from being excessively opened to prevent the lead from falling (e.g., dropping out) even when the tip chuck is releasing the lead.
Furthermore, preferably, the axial length of the holding section of the tip chuck for holding the lead inside thereof is set to be longer than that of the delivery chuck. This configuration enables the lead to be firmly sandwiched by the tip chuck, and to prevent the lead from being broken.
Additionally, preferably, a protrusion is provided at the rear end of the tip chuck, and the rear end at which the protrusion is provided, is pressed in the tip of a through-hole in an axial cylinder disposed inside the tip fitting so as to penetrate the tip of the tip fitting. A staged surface that abuts the protrusion and a larger-diameter portion that is separated from the protrusion behind the staged surface may be formed at the tip of the through-hole. The protrusion of the tip chuck engages the staged surface of the axial cylinder to prevent the tip chuck from slipping out from the axial cylinder. Additionally, since the larger-diameter portion that is separated from the protrusion of the tip chuck is formed on the axial cylinder, a radial load from the protrusion is precluded from being constantly applied to the axial cylinder, thereby to prevent the axial cylinder from being damaged.
Preferably, at least the tip chuck is formed of the synthetic resin material. This composition reduces material costs and manufacturing processes, thereby reducing manufacturing costs.
The color of the tip chuck formed of the synthetic resin material may vary depending on the diameter of the lead. This configuration enables the user to determine quickly and simply a lead diameter compatible with the mechanical pencil merely from the color of the tip chuck.
Additionally, a staged surface for abutting the tip surface of the tip fitting is provided on the tip chuck, and the rear end of the tip chuck can be coupled to the tip of a lead delivery mechanism disposed inside the tip fitting, so as to penetrate the tip of the tip fitting. When the tip fitting is disassembled from the outer tube, the staged surface of the tip chuck abuts the tip surface of the tip fitting to hinder the tip chuck and the lead delivery mechanism from slipping out from the tip fitting, thereby preventing the parts from being disconnected and lost. Since the tip chuck is formed of the synthetic resin material, the staged surface can be formed simply by molding.
Additionally, the tip chuck can be formed by combining a plurality of separate chuck members together. This configuration enables the tip chuck to be formed simply to desired sizes by molding each chuck member into a desired shape.
Additionally, the tip chuck formed of the synthetic resin material has a plurality of chuck elements that hold the lead. The plurality of chuck elements are mutually extended in the outer-diameter direction when the molding of the tip chuck is finished, and the rear ends of the chuck elements may be pressed in the tip of a central hole in the axial cylinder disposed inside the outer tube to reduce the mutual extension of the tips of the chuck elements to provide a holding force sufficient to prevent the lead from falling freely (e.g., dropping out from the pencil) when the tip chuck advances. This configuration eliminates secondary processing such as burr removal or the narrowing of the tip of the chuck to reduce manufacturing processes and costs. Even when the lead becomes shorter than the length between the tip and delivery chucks, the lead is prevented from falling because the mutual extension of the chuck elements is reduced, thereby to provide a holding force sufficient to prevent the lead from falling freely (e.g., dropping out) when the tip chuck advances to release the lead.
Additionally, optionally, a ring-like member is positioned on the outer circumferential surface of the middle of the tip chuck to limit the radial extension of the chuck elements. After the ring-like member has restricted the radial extension of each chuck element, the tip chuck can be pressed in the axial cylinder to facilitate the press-in operation. Furthermore, optionally, a protrusion that is pressed in the central hole in the axial cylinder is formed on the outer circumferential surface of the rear end of the chuck elements, and a notch for deforming the protrusion in the inner-diameter direction is formed in the side end of the chuck elements. This configuration can facilitate the operation for pressing the chuck elements in the axial cylinder.
According to another aspect of the present invention, the tip chuck formed of the synthetic resin material has a plurality of chuck elements that hold the lead. The plurality of chuck elements are mutually extended in the outer-diameter direction with respect to each other when the molding of the tip chuck is finished, and a ring-like member for hindering the radial extension of the chuck elements may be installed on the outer circumferential surface of the tip chuck to reduce the mutual extension of the tips of the chuck elements, to provide a holding force sufficient to prevent the lead from falling freely when the tip chuck advances. By forming the tip chuck of the synthetic resin material, material costs can be reduced, and the tip chuck can be molded in the desired form. This configuration also eliminates secondary processing such as burr removal or the narrowing of the tip of the chuck to reduce manufacturing processes and costs. Even when the lead becomes shorter than the length between the tip and delivery chucks, the lead is prevented from falling because the mutual extension of the chuck elements is reduced, thereby to provide a holding force sufficient to prevent the lead from falling freely when the tip chuck advances to release the lead.
Additionally, optionally, a notch for deforming the tip chuck in the inner-diameter direction is formed in the side end of the chuck elements so as to correspond to the installation of the ring-like member. This configuration facilitates the operation for installing the ring-like member on the tip chuck.
The present disclosure relates to subject matter contained in Japanese Patent Application HEI 9-122683, filed May 13, 1997 and from Japanese Patent Application HEI 9-166901, filed Jun. 24, 1997, which are expressly incorporated herein by reference in their entirety.